The sun shined with less luminescence - as much as 30 percent weaker - which meant the Earth should have been really cold. So cold, in fact, that liquid water would not have existed.

But the geologic record shows that water was, indeed, present and provided the foundation for the proverbial "primordial soup" that gave rise to life. How come? This is what's called the "faint young sun problem."

There are many theories, among them that the Earth's reflectivity was lower because of smaller continents, allowing more sunlight to be absorbed. But one of the leading theories examines the atmosphere of the Archaean period, specifically the presence of greenhouse gases like carbon dioxide and methane that might have warmed the atmosphere to temperatures at or above today's.

The same greenhouse gases that, in abundance, are getting us into trouble today, may have been fundamental to the Earth's life-creating conditions. As geochemist James Kasting of Penn State University points out in Chapter 8, The primitive Earth of Prebiotic Evolution and Astrobiology, methane and carbon dioxide should have been abundant in the first several hundred million years of Earth's history because of degassing during the planet's formation.

The concentrations would have declined over time, methane converting to carbon dioxide, and carbon dioxide converting into carbonate rocks. But the storage of carbon in rocks would have slowed with dropping temperatures.

Meanwhile, continually-spewing volcanoes would have then provided the carbon dioxide boost to the atmosphere to spike temperatures back up again. The feedback loop, called the carbonate-silicate cycle, goes on to this day.

Clearly more went on than we know. CO2 concentrations would have had to be about 10 times higher than today's values, an unlikely scenario given that past research estimates the concentration no larger than three times that of today. Methanogens, bacteria-like organisms that lived in oceans and marine sediments, may have gassed enough methane to make up at least some of the difference.

Why does this matter to modern day climate change? "One thing that paleoclimate research definitely does do is to put modern day climate change into perspective," says Kasting.

Let's compare the numbers. A 30 percent change in solar luminosity, up to today's levels, corresponds to an extra 80 watts/m2 in atmospheric heat. Every doubling of anthropogenic CO2 amounts to an extra 4 watts/m2. If we get up to 12 watts/m2, well within our capabilities, we're staring down a temperature change of between 6 to 12 degrees C. That's still nowhere near the sun's radiative forcing, but also not that far off.

Early Earth needed CO2 to warm up, but we sure don't anymore. There can be too much of a good thing.

The cold tongue
The disappearance of glaciers goes hand in hand with warming temperatures. But it turns out that the process may be more complicated than rising CO2 levels in the atmosphere. For insight, we look to the past.

The Pliocene epoch was last era in which temperatures were this warm, about 3 to 5 million years ago. CO2 concentrations were 30 percent higher, sea levels 15 to 20 meters higher, and temperatures more than 5 degrees Fahrenheit hotter. And there were no glaciers, except intermittent ice caps on Greenland.

What kept the ice sheets at bay has been explored in a recent paper in the journal Paleoceanography by UC Berkeley geographer M. Vizcaino and colleagues.

The scientists believe that during the Pliocene, a permanent "El Nino state" may have been taking place. El Nino is a familiar climate pattern that occurs every three to seven years and is marked by wacky weather patterns. Some countries become dry, others are deluged with floods and tropical storms, all due to a relaxation in trade winds across Pacific, which cut off upwelling of cool, nutrient rich waters and cause sea surface temperatures to rise.

In a permanent El Nino, sea temperatures remain constant across the Pacific, and the cold water upwelling, known poetically as the "cold tongue" goes limp. Using climate model simulations, the scientists studied what might happen during a permanent El Nino.

As a result, warmer air invades North America, Greenland, and part of Eurasia around the Black Sea, raising temperatures by as much as 9 degrees Fahrenheit. Conversely, temperatures become the coolest in much of northeast Eurasia. This overlaps nicely with where glaciers would otherwise exist, northeast Eurasia being glacier-free. The researchers write:

The climate reorganization caused by a permanent El Nino results in temperature anomalies over the northern high latitudes remarkably coincident with known locations of ice sheet growth.

There has been plenty of speculation about what caused the onset of glaciation at the end of the Pliocene. Among the ideas: dropping concentrations of carbon dioxide, greater seasonality in sea surface temperatures, or a closing Panama Isthmus that caused stronger ocean circulation and led to the higher moisture content needed to fuel ice sheet formation.

But since Vizcaino and colleagues consider temperature the most important factor in ice sheet formation, they turn again to changes in CO2 and El Nino patterns as the long term determinants. The "cold tongue" could be more important than we thought.

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